A system for monitoring an elevator door is shown in EP 1 345 444 A1, which comprises a three-dimensional semiconductor sensor and a processing device. The three-dimensional semiconductor sensor includes a light source radiating light in a specific range, wherein the light source is preferably a laser diode. The light emitted by the light source is reflected by the elevator door. A sensor group mounted in the region of the elevator door receives the light reflected by the elevator door. Further, the processing unit converts the light information into three-dimensional image information, subsequently compares the image information with a reference image or several stored referenced images and recognizes changes. In the case of a recognized change the processing unit triggers a reaction adapted to the situation.
Three-dimensional semiconductor sensors enabling three-dimensional detection of image information are complicated and costly. Moreover, in order to obtain sufficient light information for evaluating a three-dimensional image information a separate light source, which provides a high light intensity, is necessary. Further, the light source must be positioned and aligned in such a way, that the light emitted by the light source is reflected in an appropriate manner. As a further disadvantage a high amount of information needs to be analyzed and evaluated by the evaluation unit. Thus, an external microprocessor is necessary to provide a high data processing power.